The Societé Nationale des Chemins de Fer Luxembourgois (CFL) has been modernising its rolling stock and infrastructure with a significant programme at Luxembourg Gare to accommodate the high speed train from France (with Luxembourg to Paris in 2 hours 15 minutes). In addition it has been working on the links from Luxembourg Gare to the north of the country and to Germany in the east, relevant to the Pulvermühle viaduct upgrading.
The existing line from the Gare to the viaduct is in tunnel and double track which continues over the viaduct. Immediately thereafter it splits with the northern line continuing in double track and the eastern link being only in single track for some five kilometres before reverting to double track. This configuration even at the relatively low traffic levels provides a constraint to the traffic flow, particularly to Germany. In addition it is envisaged that some local traffic might use the viaduct with the development of an urban train-tram system. The general condition of the viaduct is now good having been renovated in 2003, but the single track section to the east is rather poor with some settlement.
A programme of works has been conceived to resolve these capacity and reliability concerns comprising the widening of the viaduct and the tunnel from two to four tracks, and the provision of two tracks on the link to the east for some 500 metres initially. The missing five km of double track eastwards to Trier will be upgraded under a separate contract. The project visited consisted of the tunnel works, the viaduct and the embankment stabilisation.
The tunnel widening works are in sound rock but are complicated by being relatively shallow and thus conflicting with numerous services, drains and cables as well as a main road, which requires numerous phases of diversions. It also coincides with part of the 17th century Vauban fortifications requiring special attention from archaeologists and others. Several skeletons were unearthed just outside the fortifications presumed to date from one of the sieges in the early 17th century. Beside the tunnel, a platform has been built at the viaduct level to allow the erection and launching of the new bridge deck.
The existing viaduct is a splendid 19th century construction (built by the British firm Waring Bros. in about 1855) and is some 240 metres long with maximum height of 30 metres. As has been noted, it was recently renovated with a new concrete deck, infilling of voids and some pre-stressing to stabilise the brickwork. An open design competition was held to determine the best solution for the new project and numerous ideas were put forward. UNESCO was involved in the choice as the works are in a designated heritage area. The selected project was to build a separate bridge beside the existing one but in a subservient style. It comprises pillars, partly obscured by the existing viaduct, in a black concrete with a steel truss structure above, topped off with a concrete deck. The pillars are to be supported by drilled concrete piles through to the bed rock. The overall result should be successful in harmonising well with the older iconic structure.
The embankment works are the most complicated and technically the most interesting – which surprised everyone. The existing track is perched on a 30 metre high embankment with a 45° slope, protected by stone paving. It was known that the embankment was suffering with settlement and requiring considerable ballast to maintain track stability. From old maps it was known that the river Alzette was initially close to the cliffs and was diverted to its present position during construction, with the existing track being supported entirely by the embankment. After numerous boreholes, both vertically and horizontally, it was revealed that the embankment was very poorly compacted, full of voids and that a layer of very fine silt underlay the whole.
The solution proposed was to grout this silt layer to ensure its strength and to proceed with consolidating the remaining embankment. The new double track itself has been designed to be supported by a concrete frame, supported on a block foundation, itself piled through to bed rock some 30 metres below. The full extent of these works was not appreciated initially but luckily was mainly known prior to the letting of the construction contract. Efforts have been made to limit the environmental impacts, as the valley is an attractive recreation area and forms part of an UNESCO heritage site. Some trees have been cut down but replaced by equivalent ones nearby.
The works commenced in 2008 and should be completed by 2014. The works have to be undertaken with the railway in operation which obviously complicates progress. The cost of the main works is about 130 m Euro.
The design competition was won by a German-Luxembourg group with an Italian architect. The construction group is composed of a Dutch, Belgian and Luxembourg firms.
We had an excellent guide in CFL’s chef de chantier and we hope to re-visit the site when the steel bridge sections are being winched out into position.
Peter Bond CEng MICE, ICE Representative for Luxembourg